As is customary when discoveries of this nature are made, the controlling authorities have pointed out that natamycin in wine does not constitute a health hazard, but it is prohibited to add this antibiotic to wine.
Importers of wine and wine merchants - presumably supermarkets, since these are cheap wines - had made provisions for this and are having the wines tested in private laboratories.
It is assumed that rectified concentrated grape must was treated with this antibiotic. Concentrated grape must is added to inferior wines either before fermentation or is used to sweeten the finished wine.
Please bear in mind that good wines do not require concentrated grape must, but they do cost a bit more.